X-ray Detector for Electron Microscope

ABSTRACT

Multiple detectors arranged in a ring within a specimen chamber provide a large solid angle of collection. The detectors preferably include a shutter and a cold shield that reduce ice formation on the detector. By providing detectors surrounding the sample, a large solid angle is provided for improved detection and x-rays are detected regardless of the direction of sample tilt.

This application is a Continuation Application of U.S. application Ser.No. 13/855,373, filed Apr. 2, 2013, which is a Continuation Applicationof U.S. application Ser. No. 13/312,689, filed Dec. 6, 2011, now U.S.Pat No. 8,410,439, which is a Continuation Application of U.S.application Ser. No. 12/494,227, filed Jun. 29, 2009, now U.S. Pat. No.8,080,791, which claims priority from U.S. Provisional App. No.61/122,295, filed Dec. 12, 2008, all of which are hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to x-ray detectors, and in particular, toan x-ray detector for an electron microscope.

BACKGROUND OF THE INVENTION

Electron probe microanalyzers and electron microscopes having anattached x-ray spectrometer are used to determine the composition ofmicroscopic or nanoscopic regions of a surface. The detectors determinethe energy or wavelengths of x-rays emitted from the sample and inferthe composition of material under the electron beam from the energy orwavelength of the x-rays. Detectors that use a crystal to disperse andanalyze x-rays of different wavelengths are referred to as wavelengthdispersive spectrometers (WDS) and detectors that measure the energy ofincoming x-rays are referred to as energy dispersive spectrometers(EDS). While a WDS can provide better resolution and faster counting fora particular wavelength of x-ray, an EDS is better adapted to measuringx-rays of different energies from multiple elements.

Two types of semiconductor energy dispersive x-ray detectors arecommonly used in electron microscopy: lithium-drifted silicon detectors“Si(Li)” and silicon drift detectors “SDD”. Si(Li) detectors typicallyrequire cooling to liquid nitrogen temperatures and normally have astandardized active detection of area of 10, 30 and 50 mm² SDDs canoperate at a higher temperature and can provide better resolution athigh count rates. To avoid ice formation and contamination on thedetector, as well as damage from backscattered electrons, a window of alight element such as berrylium is often attached in front of thedetector to stop the electrons. A magnetic field can also be used nearthe detector entrance to divert electrons away from the detector. Acollimator is often used in front of the detector to reduce x-rays fromsources other than the sample from entering the detector. Somedetectors, such as the one described in U.S. Pat. No. 5,569,925 to Quinnet al., include a shutter in front of the detector. When the electronmicroscope is operated under conditions that would generate high energyx-rays and electrons that could damage the detector, the shutter can beclosed to protect the crystal.

Ice formation is also reduced by providing a colder surface near thedetector. For example, in the system described in U.S. Pat. No.5,274,237 to Gallagher et al. for a “Deicing Device for CryogenicallyCooled Radiation Detector,” the heat generated by the detector circuitrymaintains the detector a few degrees warmer than the collimator surfaceso that moisture sublimes from the detector surface onto the collimatorsurface. The heat generated by the circuitry provides a temperaturedifference of only about five degrees, which may not be adequate tomaintain an ice-free surface on the detector. U.S. Pat. No. 4,931,650 toLowe et al. for “X-ray Detectors” describes periodically heating thedetector above its operating temperature while maintaining a heat sinkat operating temperature. Periodically heating the detector above itsoperating temperature does not stop the build-up of ice during operationand requires periodic interruption of the system operation to remove theice.

For greatest sensitivity, the detector should cover a large solid anglefrom the sample to collect as many of the emitted x-rays as possible. Toincrease the solid angle, the detector can provide a larger activesurface area, or be placed closer to the sample. In a transmissionelectron microscope, the pole pieces and sample holder take up most ofthe space around the sample and it can be difficult to position X-raydetectors close to the sample to increase the solid angle. U.S. Pat. No.4,910,399 to Taira et al. teaches a configuration that puts a detectorcloser to the sample and allows the detector to subtend a larger solidangle. Another configuration is shown in Kotula et al., “Results fromfour-channel Si-drift detectors on an SEM: Conventional and annulargeometries,” Microscopy and Microanalysis, 14 Suppl 2, p. 116-17 (2008).Kotula et al. describe a four-segment detector, with each segment beingkidney-shaped and having an active area of about 15 mm² The detector ispositioned above the sample below the pole piece of an SEM, with thefour segments distributed in a ring that is coaxial with the electronbeam. This configuration is not normally possible in a high-resolutionTEM.

SUMMARY OF THE INVENTION

An object of the invention, therefore, is to provide an x-ray detectorhaving improved detection capabilities.

A preferred embodiment uses multiple detectors arranged in a ring withina specimen chamber to provide a large solid angle of collection. Thedetectors preferably include a shutter and a cold shield that reduce iceformation on the detector. By providing detectors surrounding thesample, a large solid angle is provided for improved detection andx-rays are detected regardless of the direction of sample tilt.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows schematically a preferred detector of the presentinvention;

FIG. 2 shows the arrangement of detectors around a sample;

FIG. 3 shows the mechanical layout of a preferred detector embodiment;

FIGS. 4A and 4B show structures for mounting the detectors in which thetarget point of the detectors remains relatively constant as thetemperature changes; and

FIG. 5 shows how the difference in x-ray path length can be used inx-ray tomography without requiring a series of tilt images.

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment uses multiple detector assemblies arranged in aring within a specimen chamber to provide a large solid angle ofcollection. FIG. 1 shows schematically a preferred embodiment of one ofthe multiple detector assemblies 100. Each detector assembly 100includes a SDD detector 102 having an active area of preferably greaterthan 10 mm², more preferably greater than 20 mm², and even morepreferably about 30 mm² In some embodiments, each detector includes anactive area of between 50 mm² and 100 mm² The detector of FIG. 1includes a collimator 104 that prevents stray x-rays from entering thedetector, ensuring that the signal from the detector corresponds to thex-rays emitted from a sample 106. A mechanical shutter 108, shown in theclosed position, prevents electrons and low-energy x-rays from damagingthe detector when it is not in use.

The SDD detector 102 is cooled to about 200 K using liquid nitrogen andis surrounded by a cold shield 110 maintained at about 100 K. In otherembodiments, the detectors are maintained at temperatures of betweenabout −60° C. and about −80° C. for optimum detector performance.However, it is possible to operate the SDD at higher temperatures, up toand including room temperature. Harmful background gases in an electronmicroscope are mostly water vapor and hydrocarbons, such as, forexample, oils. Ice and hydrocarbons tend to condense onto the detectors,absorbing some incident x-rays and reducing the collection efficiency.Maintaining the detector cold shield at a temperature that issignificantly colder than the detector and operating in an ultra highvacuum prevents the build-up of ice on the detector. The temperaturedifference between the detector and the cold shield is preferablygreater than 10° C., more preferably greater than 25° C., even morepreferably greater than 50° C., and most preferably equal to or greaterthan about 100° C. The shutter, which is maintained at about the sametemperature as the cold shield, also protects the detector from iceformation. By reducing ice formation on the detector, a preferreddetector does not require a window. Eliminating the window improves thedetection efficiency for low-energy x-rays. Some embodiments, however,still use windows of a material, such as beryllium or thin polymerfoils, which minimize x-ray absorption. Collimators 104 preferably arein thermal contact with cold shield 110 to provide additional coldshielding.

FIG. 2 shows a preferred arrangement of detectors 202 positioned arounda sample 204 below, and thermally isolated from, an upper pole piece 206of a TEM 208 having an optical axis 209. A preferred embodiment includesfour detectors arranged around the sample, although only two are visiblein FIG. 2. The two detectors 202 are positioned on opposite sides of theeucentric sample stage tilt axis, which is normal to the plane of thedrawing. An active area 210 of each detector preferably subtends a solidangle of about ¼ steradian or greater to provide a total detector solidangle of about a steradian or greater. Each detector preferably detectsx-rays at angles of less than 50 degrees from the sample surface, morepreferably at angles of less than 35 degrees from the sample surface,and more preferably at angles between 5 and 35 degrees from the samplesurface. The low take-off angle reduces damage from backscatteredelectrons, while maintaining bremsstrahlung radiation, which also givesrise to background in the spectra, but at a reasonably low level. Thelow take-off angle detectors also require no space between the polepieces, thereby not requiring a degradation of the TEM or STEMresolution and sample tilt range. When the term “take-off angle” is usedto describe a location or orientation of a detector, it is assumed thatthe sample surface is horizontal.

FIG. 3 shows schematically a TEM 300 including four x-ray detectorsassemblies 301 (only two shown) positioned around a TEM optical axis302. Each x-ray detector assembly 301 includes an X-ray detector 303,which is preferably an SDD detector, which is known in the art. X-raydetector assemblies 301 are supported by a support ring 304 which is inturn supported by a lower pole piece 306 with a thermal insulator 305thermally isolating support ring 304 from lower pole piece 306. Theoptical axis 307 of each detector assembly 301 preferably intersects theoptical axis 302 of the TEM at the sample surface. Thermal conductor310, preferably a copper braid or rod, connects support ring 304 with acold source, such as a Dewar flask 312 of liquid nitrogen maintainedoutside of vacuum chamber walls 313. A support 314 provides support toeach detector assembly 301 from support ring 304 and provides aresistive thermal path from support ring 304 to detectors 303, that is,support 314 provides a thermal path between support ring 304 anddetector 303, but restricts the thermal flow. A resistive heater 316maintains the temperature of each detector 303 significantly greaterthan the temperature of support ring 304. Detectors 303 are preferablymaintained at a temperature difference of greater than 20° C., greaterthan 50° C., or approximately equal to or greater than 100° C., relativeto support ring 304. A collimator is formed by an upper collimator 318and a portion of support ring 304. The use of an SDD detector, which canoperate efficiently at a temperature significantly higher than theoptimum operating temperature for a Si(Li) detector, allows the detectorto be operated at a temperature significantly above the temperature ofliquid nitrogen. This facilitates the use of a single liquid nitrogencold source to cool both the detector and a cold shield, while allowingthe cold shield to be maintained at a much lower temperature than thedetector, which improves protection of the detector from contaminationby condensates.

Each detector 303 includes an active area 320 positioned on a ceramicsubstrate 322 supported by a metal base 324. Four shutters 330 protectthe four detectors 303 from ice formation and from high energy electronswhen closed. During operation, the shutters are moved to an openposition to allow x-rays to reach the active area 320. The shutters arein thermal contact with support ring 304, which keeps them at about thesame temperature as support ring 304 and at a temperature significantlylower than the temperature of detector 303. Support ring 304, shutters330, and preferably upper collimator 318, function as a cold shield,causing moisture to sublime from the detector active area 320 onto thesupport ring 304, shutters 330, and collimator 318. The shutters arecontrolled by a shutter controller 331 through a shutter activator 332which connects to the shutters through a mechanical linkage via a vacuumfeed-through 333 from outside the vacuum chamber walls 313. In oneembodiment, each shutter activator activates two shutters. TEM 300 iscontrolled by a user 340 through the TEM Personal computer 342.

A heater controller 334 controls the current to heater 316 to maintain adesired temperature of detector active area 320. A pre-amp 338 receive asignal from the detector 303. The signal is processed by a pulseprocesser 337 to determine the number of x-rays counted and the energyof each x-ray. Pulse processing techniques are well known in the EDSart.

Prior art detectors were typically attached to the vacuum chamber wall,more than 10 cm away from the sample, which made it difficult to keepthe detectors aligned with the sample. When a detector is even 0.2 mmout of position, it is no longer “looking” at the point at which thex-rays are generated, and therefore picks up unwanted signals. Inpreferred embodiments of the present invention, the detector assembly isattached within the TEM lens assembly, preferably to the lower polepiece. Because the detector is typically cooled to cryogenictemperature, it would have been considered undesirable to attach thedetector assembly to the pole pieces, because it could lead to thermalinstability. Applicants have found, however, that insulation 305 betweenthe detector assembly and the pole pieces reduces thermal instabilities.The detector can also be secured to or against an upper pole piece 346to provide additional alignment and stability. Moreover, the electricfield from the detector, particularly a windowless detector, affects theelectrons in the primary beam. Applicants have found that any deflectionin the primary electron beam is relatively small and that the improvedaccuracy of the detector positioning outweighs any disadvantages of themounting. Mounting the detector assembly onto the lens provides improvedmechanical accuracy relative to the specimen.

FIGS. 4A and 4B show two designs for supporting a detector support ring402 that in turn supports at least two individual detectors assemblies.Each design is shown in a front view and a top view. Both designssupport the detector support ring using flexible elements 400, such asleaf springs, that are connected on the one end to the detector supportring and on the other end to a support, such as a portion of the lens.The detector support ring is preferably supported within the TEM lens,preferably on the lower pole piece (not shown). The flexible elementsprovide a floating thermal and vibration isolation platform on which tomount the detectors.

It is desirable that the detectors maintain their aim on theintersection of the optical axis and the sample surface as the detectorsare cooled to operating temperature so that the detector can be alignedat room temperature and then stay aligned as it is cooled. In the designof FIG. 4A, the thermal center, that is, the point on the sample beinganalyzed, preferably does not move as the temperature of the detectorchanges. That is, the detectors are always ‘looking’ at approximatelythe same point: the intersection of the sample with the axis of themicroscope as the temperature changes. The symmetry of the four leafsprings provides a slight rotation about the microscope axis as theassembly is cooled to operating temperature, as well as a radialcontraction. The material of the leaf springs and the construction ofcomponents are selected to provide thermal coefficients of expansionthat provide thermal stability such that the thermal center of thedetectors on the microscope axis changes by less than 10 μm when theassembly is cooled from room temperature to cryogenic temperatures, suchas to 100 K for the support ring. The thermal expansion coefficients ofthe different materials used to support the detector support are matchedto achieve a net near-zero displacement of the thermal center. Thethermal gradient in the flexible elements during operation is preferablylinear, while the thermal gradient in the detector support ring isminimal The mass and the dimensions of the leaf springs raise theresonance frequency of the mount to minimize vibration. While leafsprings are described to float the detector mount, other types of mountscan also be used.

In the design of FIG. 4A, a change in temperature causes expansion ofthe flexible elements, which causes a slight rotation about the point TCon the optical axis and a radial contraction. The rotation about thepoint TC is not detrimental since x-ray emission is isotropic in theazimuthal direction. In the design of FIG. 4B, a change in temperaturecauses translation parallel to the optical axis, which is lesspreferred.

The support ring may include an opening that accepts the upper polepiece of a TEM and a thermal isolation material thermally isolates thecold components from the upper pole piece. Openings in the ring supportcan be provided for inserting the sample, an aperture, or other devices.By mounting the support ring on the lower pole piece, the positioning ofthe x-ray detectors relative to the sample is maintained more accuratelythan the positioning of prior art detectors, which are mounted onto thewalls of the vacuum chamber.

The invention allows three-dimensional X-ray tomography and depthdetermination of features in the specimen without requiring a series ofimages at different tilt angles. In the prior art, X-ray tomography wasperformed by obtaining a series of images at different sample tilts. Theimages at the different tilts were analyzed by a computer to determinethe three dimensional structure of the sample region. Because thepresent invention provides multiple detectors, it is possible to use thedifference in signal strengths between the detectors to determine thedepth of a feature or the three dimensional distribution of materials.

FIG. 5 shows a defect 500 in a sample 502 under an electron beam 604.X-rays emitted from the defect travel through the sample a distance 510before reaching detector 512. X-rays emitted from the defect travelthrough the sample a distance 514 before reaching detector 516. Thex-rays are attenuated as they travel through the sample, and so thex-ray signal at detector 512 will be different from the x-ray signal atdetector 516. The X and Y coordinates of the point at which the x-raysare generated is known from the known position of the scanned image. TheZ position of the point from which the x-rays are generated can then bedetermined from the differences in signal strength at the differentdetectors, based on the different path lengths through the samplematerial.

Another use of the multiple detectors is differential x-ray detection,i.e. subtracting the signal from the 4 detectors in some combinationslike (A+B)−(C+D). This could be used to detect local differences inmaterial properties or magnetic anisotropy, for example, in addition tothe tomography application.

Conventional x-ray tomography is enhanced through the use of multipledetectors by increasing signal acquisition rates and/or by combininginformation from a tilt series with information from difference insignal intensity, as described with respect to FIG. 5, from the multipledetectors

Embodiments of the invention substantially increase X-ray count ratescompared to most prior art detectors. Unlike the prior art, in which thesample needs to be tilted toward the x-ray detector, embodiments of theinvention allow detection at any tilt angle of the specimen underobservation or at any stage rotation, because the detectors surround thesample. In some embodiments, the functionality of the cold trap andcooling for the detectors is combined in one liquid nitrogen Dewar,rather than the two Dewars normally required, thereby reducing thechance of adverse effects on the image resolution of the microscope. Themultiple detectors and the large solid detection angle, preferably aboutone steradian or greater, allows X-ray mapping, which previouslyrequired more than 1 hour measurement time, to be performed in a fewminutes.

The invention also leads to new possibilities for X-ray detection, suchas 3D X-ray tomography and depth determination of features in thespecimen. Here the independent directional detection of the X-raysreceived by the multiple detectors carry the information about the exact3D position of the area on the sample emitting the X-rays.

While the embodiments described above describe the implementation ofx-ray detectors for a transmission electron microscope, the invention isnot limited to implementation in a TEM, but can be implemented in otherinstruments, such as scanning electron microscopes and scanningtransmission electron microscopes.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable.

The drawings are intended to aid in understanding the present inventionand, unless otherwise indicated, are not drawn to scale.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows:
 1. An x-ray detector assembly for an electronmicroscope, comprising: a mount configured for being supported withinthe lens of a transmission electron microscope; at least two x-raydetectors supported by the mount, the x-ray detectors arranged toreceive x-rays from at least three different directions from a sample,each detector including; a cold shield at least partly surrounding thedetector; and a heat source to maintain the detector at a highertemperature than the cold shield. 2-35. (canceled)